TORONTO, Nov 29 (Reuters) - Canada's main stock index ticked lower on Wednesday, hurt by a sharp fall in the shares of Canadian National Railway Co, while an earnings beat helped push Royal Bank of Canada higher.

* CN Rail fell as much as 4.9 percent, to its lowest level since March, and was last down 2.8 percent at C$97.18. Its main rival, Canadian Pacific Railway Ltd, said it was adding several senior sales executives.

* The materials group, which includes precious and base metals miners and fertilizer companies, lost 0.7 percent as prices for gold as well as copper, aluminum and other base metals fell.

* Potash Corp added 0.9 percent to C$25.16 after its chief executive said he expects a "very tight" balance of potash supply and demand in 2018 and that the company had received "broad interest" from Chinese buyers interested in its stake in a Chilean lithium producer.

* Six of the index's 10 main sectors were lower, with decliners and advancers evenly matched overall. (Reporting by Alastair Sharp; editing by Jonathan Oatis)